Solvent-based coating compositions are known which can be applied to a substrate, for example, by spraying, and then cured by baking the coated substrate at an elevated temperature suitable to drive off the organic solvent and to promote crosslinking reaction. The resulting thermoset coating, if sufficiently humidity and solvent resistant, can provide aesthetic and functional advantages including corrosion protection for the underlying substrate.
Automobile manufacturers, in their efforts to extend the expected life of automobile sheet metal and the like, have directed considerable attention to various processes and compositions designed to result in not only improved corrosion resistance but also improved chip resistance properties. In particular, research and development efforts have recently been directed to obtaining primer compositions which are flexible and chip resistant and which give corrosion protection while exhibiting good humidity and solvent resistance, as well as good intercoat adhesion. New automobile designs and concern about chipping in areas exposed to stones, gravel and other road debris, e.g. rocker panels, have demanded such chip resistant primers which can be applied in reasonable thicknesses by techniques which do not require extensive and expensive process modifications during painting operations. To date available primers, whether high or low solids, have not proven suitable.
In order to overcome the aforementioned chipping problem it has been common to apply relatively thick chip resistant coatings in body panel regions which are inclined to chip, prior to application of still another primer composition. One such chip resistant sealer material which has been employed is a polyvinyl chloride plastisol sealer which has been applied with airless spraygun equipment in thicknesses of about 20 mils in regions subject to high levels of chipping. Problems attendant with such thick coatings are readily apparent. Because of the thickness in the region to which it is applied, these materials present an appearance problem often resulting in waviness and roughness in the final coating on the sheet metal. Often times surface imperfections also result from the fact that a primer is applied over the top of this sealer, with the primer and sealer being cured together. As a result some solvent and plasticizer tend to be driven out of the polyvinyl chloride plastisol and this results in a wavy and rough surface. Still further problems associated with the use of such polyvinyl chloride plastisol sealers and the like involve application technique. Since polyvinyl chloride plastisol sealers and the like must be applied in thicknesses of 20 mils of greater in order to obtain good adhesion, they cannot be feathered down to blend into other regions of the sheet metal which do not require the additional chip protection. Thus, the materials must be applied using a masking technique, whereby those regions which are not to be coated with the sealer material are masked in a separate operation prior to application of sealer. This masking is then removed after the sealer is applied. It would obviously be desirable to eliminate these additional steps in the application of the chip resistant sealer material.
Accordingly, it is a preferred object of this invention to provide a novel solvent based, thermosetting coating composition adapted for use as a chip resistant primer, which primer may be applied in thicknesses of less than 20 mils and which may be feathered in such a manner as to blend with paint in other areas of the substrate which do no require chip resistant coating.
It is a further object of the present invention to provide novel resins suitable for use in solvent-based thermosetting coating compositions. In this regard, it is a particular object of the invention to provide novel, hydroxy functional epoxy-polyester graft copolymer resins which are crosslinkable during cure, on the surface of a substrate.
It is another object of the invention to provide novel coating compositions which comprise crosslinkable hydroxy functional epoxy-polyester graft copolymer and blocked polyisocyanate crosslinking agent and which provide high crosslinking efficiency and tough, well cured films at minimum bake temperatures, such as when applied as automotive primers. In this regard, it is a particular object of the invention to provide a novel hydroxy functional epoxy-polyester graft copolymer resin/blocked polyisocyanate thermosetting coating composition of sufficiently low Volatile Organic Content (VOC) to aid in meeting governmental emissions guidelines and yet which can be applied to a substrate by spraying or other known method.
It is still another object of the invention to provide a composition which will form a coating on a substrate, which coating has advantageous physical properties including, for example, humidity and solvent resistance, flexibility and corrosion protection for the underlying substrate.
Additional aspects and advantages of the invention will be apparent from the following description thereof.